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Subject: Re: [boost] Numeric constants
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2009-01-30 10:23:34

John Maddock wrote:
> template <class T> T pi();

I have a fixed-point class. What actually happens when I call
pi<fixed>() ? Presumably the implementation just does "return
3.1415...", so fixed::fixed(double) gets called. Yes my fixed class
has a constructor that takes a double, and if I'm really lucky it may
do the conversion at compile time - but more likely it will happen at
run time. I would get the same code if you just declared "const double
pi = 3.1415" (or #defined it).

My 64-bit fixed point type can store more digits of PI than a 64-bit
floating point value. But I'm not aware of any way of filling those
extra digits [ignoring the existence of long double]. Similarly if I
had an arbitrary precision (i.e. variable length) type, how would it be
constructed from the 100-digit constants that you mention without
support from the language (or maybe macros)?

(As for the actual constants, the only ones I find myself using are pi
and the approximate radius of the earth.)

Cheers, Phil.

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